Surface-treated steel sheet, organic resin-coated metal container and process for producing surface-treated steel sheet

ABSTRACT

A surface-treated steel sheet and a process for its production. The surface-treated steel sheet has, on at least one surface of the steel sheet, a surface-treating layer that chiefly comprises zirconium and oxygen, and contains fluorine, the surface-treating layer containing an element of the Group II on the surface side thereof. The surface-treated steel sheet features excellent adhesion to the organic resin coating, excellent corrosion resistance, and can provide metal cans that feature excellent adhesion to the resin on the inner and outer surfaces of the cans and excellent resistance against the elution of fluorine.

TECHNICAL FIELD

This invention relates to a surface-treated steel sheet, an organicresin-coated surface-treated steel sheet, a metal container, and aprocess for producing the surface-treated steel sheet. Morespecifically, the invention relates to a surface-treated steel sheetfeaturing excellent adhesion to the organic resin coating and excellentresistance against the elution, and to a process for producing the same.

BACKGROUND ART

The treatment with the chromate has heretofore been known as a treatmentfor improving close adhesion between a steel sheet and an organiccoating in the field of domestic appliances, building materials,vehicles, aircraft, containers and the like, and has, therefore, beenwidely employed owing to its excellent corrosion resistance and closeadhesion.

The treatment with the chromate pertains to either the one of the typeof containing hexavalent chromium in the coating and the one of the typeof containing no hexavalent chromium in the coating. In recent years,however, from the standpoint of environment and health in the workingenvironment, it is a growing trend to ban the use of any startingmaterial that contains hexavalent chromium irrespective of the state ofthe final products.

Materials for producing metal containers such as cans and lids aretreated with the chromate of the type that does not leave hexavalentchromium in the final products, as a matter of course. Besides, thematerials are, usually, coated with an organic resin. For instance, thetin-plated steel sheet is cathodically electrolyzed in an aqueoussolution of sodium bichromate, a steel sheet is cathodicallyelectrolyzed in an aqueous solution of a fluoride-containing chromiumanhydrite, and an aluminum alloy is treated with a chromic phosphate,followed, further, by the coating with an organic resin.

Metal containers such as cans and can lids are, in many cases, subjectedto the retort treatment with hot water in order to sterilize thecontents. Therefore, the materials are subjected to a severe environmentarousing a problem of decrease in the adhesion between the resin coatingand the surface of the metal. To solve the problem, therefore, variousstudies were so far made. At present, in order to improve close adhesionduring the treatment with hot water, the tin-plated sheet and the steelsheet electrolytically treated with chromate that are used as materialsfor producing cans, are washed with warm water or hot water in the stepof finally treating the surfaces. Namely, anions such as sulfuric acidions and fluorine ions in the treated coating are controlled to obtain ametal surface that features excellent adhesion to the organic coating(non-patent document 1, patent document 2).

As the chromium-free surface treatment studied in recent years inconnection with the steel sheets, there has been proposed a diptreatment using a treating liquid that contains Zr (zirconium) or Ti(titanium) (patent document 1). However, the steel sheet treated for itssurface by being dipped in the Zr- or T-containing solution has poorcorrosion resistance in the coating thereof. Besides, the rate ofdepositing the coating is small as compared to the electrolyticchromate-treated steel sheet (TFS) that has heretofore been used as amaterial for producing cans. Therefore, the productivity is very poor.As a high-speed treatment to substitute for the dip treatment,therefore, there have been proposed a Zr and/or Ti treatment and/or anAl treatment by applying the cathodic electrolysis. It has been knownthat both of these treatments are capable of forming an oxygen compoundof a metal on the surface of the base material at high speeds, (patentdocuments 3, 4 and 5).

As a method of improving close adhesion of a coating of an oxygencompound of a metal to an organic resin layer, further, there has beenproposed a technology concerned to a method of producing a steel sheetfor containers having a conversion-treated film that contains metal Zrin an amount of 1 to 100 mg/m² and F in an amount of not more than 0.1mg/m² by forming, on the base material, a coating of an oxygen compoundof a metal that contains an oxygen compound of Zr, and washing thesurface of the coating of the metal oxygen compound with hot water ofnot lower than 80° C. (patent document 6).

PRIOR ART DOCUMENTS Patent Documents

-   Patent document 1: International Laid-Open WO2002/103080-   Patent document 2: JP-A-7-11483-   Patent document 3: JP-A-2004-190121-   Patent document 4: JP-A-2005-97712-   Patent document 5: JP-A-2006-348360-   Patent document 6: International Laid-Open WO2012/036200

Non-Patent Document

-   Non-patent document 1: “History of Coated Steel Sheets for Cans in    Japan”, Foundation, Japanese Association of Steels, published Oct.    31, 1998, page 87, last line to page 90.

OUTLINE OF THE INVENTION Problems that the Invention is to Solve

If it is attempted to improve the corrosion resistance of a metallicbase material without forming a metal-plated layer thereon but, instead,directly forming a coating of an oxygen compound of a metal comprisingchiefly an oxygen compound such as of Zr, Al or Ti on the surface of themetallic base material, it becomes necessary to increase the thicknessof the coating (amount of coating) as compared to that of themetal-plated layer. Specifically, in the use for seamless cans that areworked to a large degree, the underlying iron may be exposed due to theworking or the adhesion to the organic resin may decrease. Therefore, ithas been urged to maintain corrosion resistance by increasing the amountof the coating and, at the same time, to improve close adhesion to theorganic resin.

In addition to the above items related to close adhesion, there stillremains another problem which the present invention is to solve, i.e.,to prevent the components constituting the metal container from elutingout into the content. For the metal containers, it is very important tomaintain the quality of the content and, therefore, special attentionmust be paid to the components that may elute out into the content fromthe metal container. In general, elution of metallic components of thecontainer can be represented by elution of iron due to corrosion andelution of anions such as sulfuric acid ions and fluorine ions in thecoating. Therefore, attention must be paid to the amount of the coatingin the metal surface treatment, surface state, and adhesive force to theorganic resin coating such as film or coating, in addition to payingattention to the pH of the content and the sterilizing conditions.

A patent document 2 is disclosing an example of improving the closeadhesion by washing the surface of the coating of an oxygen compound ofa metal on the metal-plated layer with hot water. However, in case it isrequired to form the coating in a large amount as described above, itwas discovered that the washing with hot water, that has heretofore beenused for the electrolytic chromate-treated steel sheets, is not enoughfor attaining the surface properties and for suppressing the elution asdesired. Therefore, if the electrolytic chromate-treatment line isapplied to the formation of the coating of the oxygen compound of ametal, the washing must be conducted for further longer periods of timethan the conventional methods. It was, therefore, learned that therestill exist many problems such as an increase in the load in connectionwith the production and an increase in the amount of energy consumption,imposing limitation on the speed for operating the surface-treatmentline, requiring an increased number of the tanks for washing, andincreased amount of hot water that must be used.

The present invention, therefore, was contrived in view of suchcircumstances, and its object is to provide a surface-treated steelsheet which, when an organic resin layer is formed on the surfacesthereof, features excellent adhesion to the organic resin layer andexcellent corrosion resistance and an organic resin-coatedsurface-treated steel sheet, to provide an organic resin-coated metalcontainer featuring excellent adhesion to the resin on the inner andouter surfaces of the can and excellent resistance against the elutionof fluorine, and to provide a process for producing the surface-treatedsteel sheet.

Means for Solving the Problems

According to the present invention, there is provided a surface-treatedsteel sheet having a steel sheet and a surface-treating layer on atleast one surface of the steel sheet, the surface-treating layerincluding zirconium, oxygen and fluorine, wherein said surface-treatinglayer contains an element of the Group II on the surface side thereof.

In the surface-treated steel sheet of the present invention, it isdesired that:

(1) The element of the Group II is present as a fluorine compound;(2) The element of the Group II is at least either calcium or magnesium;(3) A molar ratio AE/Zr of the element (AE) of the Group II andzirconium (Zr) in the surface-treating layer is not less than 0.2; and(4) The thickness of zirconium in terms of weight in thesurface-treating layer is 100 to 200 mg/m².

According to the invention, further, there is provided an organicresin-coated surface-treated steel sheet obtained by forming an organicresin coating on the surface-treated steel sheet.

According to the invention, further, there is provided a metal containeror a can lid made from the organic resin-coated surface-treated steelsheet.

According to the invention, further, there is provided a process forproducing a surface-treated steel sheet having a steel sheet and asurface-treating layer on at least one surface of the steel sheet, thesurface-treating layer including zirconium, oxygen and fluorine, saidprocess comprising the steps of:

forming a coating by cathodically electrolyzing the steel sheet in anaqueous solution that contains Zr ions and F ions; and

thereafter, adjusting the surfaces by conducting any one or more of adip treatment, a spray treatment or a cathodic electrolytic treatment byusing an aqueous solution that contains an element of the Group II foradjusting the surface.

In the process for producing the surface-treated steel sheet of thepresent invention, it is desired that:

(1) The element of the Group II is at least one of calcium or magnesium;and(2) In the step of adjusting the surfaces, a reduction ratio of fluorinefrom that of the step of forming the coating is not more than 30%.

Effects of the Invention

The present invention is capable of providing a surface-treated steelsheet which, when an organic resin layer is formed on the surfacesthereof, features excellent adhesion to the organic resin layer andexcellent corrosion resistance and an organic resin-coatedsurface-treated steel sheet, is capable of providing, as a container, anorganic resin-coated metal container featuring excellent resistanceagainst the elution of fluorine, excellent adhesion to the organic resinand excellent corrosion resistance and is, further, capable of providinga process for producing the surface-treated steel sheet.

In the present invention, specifically, the element of the Group II and,specifically, a fluorine compound of the element of the Group II is madepresent in the surface-treating layer on the surface side thereof.Therefore, fluorine is insolubilized and is suppressed from eluting out.Besides, the structure of the surface-treating layer is stabilizedwithout permitting zirconium to be dissolved, and defective portions canbe reduced over the whole surface-treating layer.

Further, the organic resin layer that is formed on the surface can beeffectively prevented from peeling when it is subjected to the workingor the heat treatment. Therefore, the invention provides thesurface-treated steel sheet which does not easily corrode even in casethe organic resin layer is cracked and the metal surface is exposedunder wet environment and which suppresses the elution of the metalcomponents constituting the container, provides the organic resin-coatedmetal container using the surface-treated steel sheet, and provides theprocess for producing the surface-treated steel sheet.

In the process for producing the surface-treated steel sheet of theinvention, the step of adjusting the surface executes at least any oneor more of the dip treatment, spray treatment or cathodic electrolytictreatment by using the aqueous solution that contains the element of theGroup II for adjusting the surface. Therefore, the electrolyticchromate-treated steel sheet that was so far washed with hot water cannow be washed with warm water or water of normal temperature, making itpossible to shorten the time for treatment as compared to when thewashing was conducted with hot water only and reduces the energyrequirement.

Moreover, by employing the step of adjusting the surface by using theaqueous solution containing the element of the Group II for adjustingthe surface, fluorine in the coating is not removed and discharged intoenvironment unlike that of when the steel sheet was washed with hotwater but, instead, fluorine in the coating is reacted with the elementof the Group II so as to be insolubilized in the coating. After the stepof forming the coating, therefore, the reduction ratio of fluorine inthe surface-treated steel sheet is suppressed to be not more than 30%,the fluorine concentration in the drain water is decreased, and adecreased load is exerted on the drain water. Thus there is provided theprocess for producing the surface-treated steel sheet while excellentlymaintaining the environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing changes in the atomic concentration in asurface-treating layer on a surface-treated steel sheet of the presentinvention in the direction of depth as found by using an X-rayphotoelectron spectrometer.

FIG. 2 is a diagram showing changes in the atomic concentration in asurface-treating layer on a surface-treated steel sheet of ComparativeExample 4 in the direction of depth as found by using the X-rayphotoelectron spectrometer.

FIG. 3 is a view schematically illustrating the sectional structure ofthe surface-treated steel sheet of the present invention.

MODES FOR CARRYING OUT THE INVENTION

The surface-treated steel sheet of the present invention has, on atleast one surface of the steel sheet, a surface-treating layer thatchiefly comprises zirconium and oxygen, and contains fluorine, whereinan important feature resides in that an element of the Group II iscontained in the surface-treating layer on the surface side thereof.

In the surface-treated steel plate of the invention, the element of theGroup II and the fluorine compound contained in the surface-treatinglayer on the surface side thereof can be confirmed relying on variouskinds of surface analyses or sectional analysis such as X-rayphotoelectron spectrometry (XPS), Auger electrophotometry (AES),analytical electron microscope (SEM, TEM) or the like.

As described above, the surface-treated steel sheet 1 obtained by thepresent invention, as shown in FIG. 3, has a surface-treating layer 3 onat least one surface (both surfaces in the drawing) of a steel sheet 2,the surface-treating layer 3 containing an element of the Group II and,specifically, a fluorine compound of the element of the Group II on thesurface side thereof. Usually, an organic resin layer is formed on thesurface-treating layer 3 to obtain an organic resin-coatedsurface-treated steel sheet which is then used as a material of metalcontainers such as cans and the like.

Described below are the surface-treated steel sheet of the invention,the organic resin-coated container using the surface-treated steelsheet, and the process for producing the surface-treated steel sheet.

(Surface-Treated Steel Sheet)

The surface-treating layer is formed on the surface-treated steel sheetof the invention, comprises chiefly zirconium and oxygen, containsfluorine, and is considered to assume a non-crystalline structure likeZrO_(x) (OH)_(y-z)F_(z). The coating dehydrogenates upon drying andfiring, dispels F, turns into an oxidized coating having muchcrystalline components and, if further heated, finally becomes a coatingof almost ZrO₂. However, the heating in excess of thermal hysteresiswhich the ordinary can materials receive induces cracks in the coatingdue to a change in the structure and renders the coating to become morelike ceramics, inviting not only a decrease in the workability but alsoa decrease in the adhesiveness to the resin coating, which is notdesirable. Further, if the amount of F extremely decreases in thesurface-treating layer due to washing With hot water or the like, thenthe structure of the coating tends to be easily changed even by theheating of a slight degree, inviting a decrease in the cohesive force ofthe coating, inducing a decrease in the corrosion resistance of theresin-coated metal sheet in the cross-cut testing, and causing corrosionor decrease in the adhesion in case the can body has received shocks.

It is, therefore, desired that the surface-treating layer maintains thestructure like ZrO_(x) (OH)_(y-z)F_(z) that contains F and OH.

The present inventors have studied over extended periods of time aboutthe relationships among the coating components such as amount of Zr,amount of F, cross-cut corrosion resistance after the retort treatment,and adhesiveness to the coating resin. As a result, the inventors havediscovered that a surface-treating coating containing much zirconium andfluorine is effective in attaining the above properties.

Concerning fluorine, however, if the surface-treated steel sheetcontaining F in very large amounts in the coating is used for producingmetal cans, then fluorine that is abundantly present in thesurface-treating layer elutes out into the content when the can isretort-sterilized or is stored at high temperatures, and may spoil thetaste of the content. On the other hand, if fluorine in the coating isforcibly removed by, for example, washing with hot water, then thesurface-treated steel plate is placed in a state that may easily inducea change in the structure of the coating, causing a decrease inproperties such as corrosion resistance and close adhesion. In dealingwith the surface-treated steel sheet of the present invention,therefore, suppressing the elution of fluorine from the surface-treatingcoating by the treatment for adjusting the surface, is effective in bothmaintaining the taste of the content and maintaining properties of themetal cans.

On the other hand, if the amount of zirconium is small, defective partsare much present in the surface-treating coating; i.e., the coatingeasily permits iron constituting the base material to elute out. Ironelutes out in the anodic reaction. Due to the cathodic reaction which isthe counter-reaction thereof, however, an alkali forms on the interfacebetween the coating resin and the metal coating. The alkali that isformed accelerates the elution of fluorine from the surface-treatingcoating and becomes a cause of interfacial peeling between the coatingresin and the surface-treating layer. Therefore, it is desired to use asurface-treating coating that contains zirconium much from thestandpoint of cross-cut corrosion resistance after the retortsterilization and close adhesion to the coating resin.

In the surface-treated steel sheet of the present invention, fluorine inthe surface-treating coating reacts with the element of the Group II inthe aqueous solution for adjusting the surface so that a fluorinecompound in which fluorine is insolubilized is formed on the surfaceside. Therefore, there is produced an effect of suppressing the elutionof fluorine as well as an effect of reducing the defective portions inthe surface-treating layer as will be described later. Accordingly, thesurface-treated steel sheet of the present invention is capable ofmaintaining corrosion resistance and close adhesion despite the amountof Zr is smaller than those of the prior art.

As for the amount of coating in the surface-treating layer on thesurface-treated steel sheet of the present invention, it is desired thatthe amount of Zr is in a range of 10 to 350 mg/m². If the amount of Zris less than 10 mg/m², it becomes difficult to attain the cross-cutcorrosion resistance after having been coated with the organic resin orto attain the close adhesion of the resin to the inner and outersurfaces of the cans to a sufficient degree. On the other hand, use ofZr in amounts in excess of 350 mg/m² is not economical and, besides,causes a decrease in the close adhesion correspondingly during theworking, and is not desirable.

The amount of F is desirably from 0.3 to 30 mg/m². If the amount of Fexceeds 30 mg/m², it becomes difficult to suppress the elution offluorine despite of forming a layer of a compound of the element of theGroup II on the surface side. If the amount of F is less than 0.3 mg/m²,on the other hand, the cohesive force of the coating decreases due to achange in the structure being affected by hydration causing, therefore,a decrease in the close adhesion and corrosion resistance, which is notdesirable.

As the element of the Group II contained in the surface of thesurface-treating layer by the treatment for adjusting the surface, therecan be exemplified beryllium, magnesium, calcium, strontium, barium andradium. Among them, however, it is desired to use Ca and Mg from thestandpoint of safety, sanitation, availability and cost in addition toforming a sparingly soluble compound upon the reaction with fluorine,and it is most desired to use Ca.

Specifically, if the amount of Zr is large, then the coating containsfluorine much and the resistance against the elution of fluorine becomesmore important. It is, therefore, desired that the molar ratio AE/Zr ofthe element AE of the Group II such as calcium and zirconium in thesurface-treating layer is not less than 0.2 and that the thickness ofthe coating in terms of the weight of the element AE of the Group IIsuch as calcium is in a range of 7 to 150 mg/m². If the thickness issmaller than 7 mg/m², the effect is small for suppressing the elution offluorine and for decreasing the surface defects. If the thicknessexceeds 150 mg/m², the cohesive force of the coating decreases,workability decreases and close adhesion decreases, which is notdesirable. If there are contained a plurality of kinds of the elementsof the Group II, AE represents the total amount thereof.

The surface-treated steel sheet of the present invention has thesurface-treating layer that chiefly comprises zirconium and oxygen, andcontains fluorine, wherein an important feature resides in that thesurface-treating layer has a layer of a compound of an element of theGroup II formed on the surface side thereof.

FIG. 1 is a diagram showing changes in the atomic concentration in asurface-treated steel sheet of the present invention in the direction ofdepth, the surface-treated steel sheet having a layer of a compoundchiefly comprising calcium and fluoride formed on the surface side ofthe surface-treating layer that is obtained by treating asurface-treated steel sheet having a surface-treating layer that chieflycomprises zirconium and oxygen and contains fluorine with acalcium-containing aqueous solution through the step of adjusting thesurface.

In FIG. 1, peaks of C1s, O1s, F1s, Fe2p3, Zr3d, and Ca2p3 were measuredby using the X-ray photoelectron spectrometer (hereinafter referred toas XPS), the sum of these elements were regarded to be 100%, and thedepth of Ar sputtering (calculated as SiO₂) from the surface wasrepresented on the abscissa to express changes in the atomicconcentrations. For reference, further, FIG. 2 shows the results of theprofiles of atomic concentrations in the direction of depth examined byusing the surface-treated steel sheet of before being treated with thecalcium-containing aqueous solution through the step of adjusting thesurface. FIG. 1 represents the analytical results of the surface-treatedsteel sheet prepared in Example 11 appearing later and FIG. 2 representsthe analytical results of the surface-treated steel sheet prepared inComparative Example 4.

It will be learned from FIG. 1 that in the surface-treated steel sheetof the present invention, Ca is present in the surface-treating layer onthe surface side thereof. As compared to FIG. 2 that is not conductingthe step of adjusting the surface, the surface-treating layer in FIG. 1has an increased F concentration on the surface side thereof but,contrary, has a decreased Zr concentration and a decreased Oconcentration on the surface side. This is presumably due to thatthrough the treatment with the calcium-containing aqueous solution,fluorine and calcium reacts with each other, and an insoluble compoundis formed in the surface. In FIG. 1, further, the atomic concentrationof Fe rises little in comparison to FIG. 2 despite the surface-treatinglayer is sputtered deep from the surface thereof and it is, therefore,learned that the surface-treating layer has a structure that permitsdefects to be little exposed.

The similar trends are exhibited in Examples 1 to 15 and ComparativeExamples 1 to 5 appearing later. Namely, in Examples, the elements ofthe Group II are present on the surface side like in FIG. 1 indicatingan increase in the F concentration on the surface side as compared toComparative Examples represented by FIG. 2.

(Method of Producing Surface-Treated Steel Sheets) <Step of Forming theCoating>

In the process for producing the surface-treated steel sheet of theinvention, first, the steel sheet, in the step of forming the coating,is cathodically electrolyzed in an electrolytic treating liquid which isan aqueous solution containing Zr ions and F ions so that the coating ofa Zr compound chiefly comprising zirconium and oxygen and containingfluorine is formed on at least one surface of the steel sheet in such afashion that the amount of Zr is in a range of 10 to 350 mg/m² and, morepreferably, 10 to 200 mg/m² and the amount of F is in a range of 0.3 to30 mg/m². Specifically, in the step of adjusting the surface that willbe described later, the reduction of fluorine can be greatly suppressedif the amount of the coating is large. Therefore, it is desired that theamount of Zr is not less than 100 mg/m² and, specifically, is in a rangeof 100 to 200 mg/m².

The steel sheet after the surface-treating layer has been formed thereonis squeezed through the rolls to remove the electrolytic treatingsolution, washed with water, further, squeezed through the rolls toremove the washing water, and is sent to the next step of adjusting thesurface.

In the electrolytic treating solution used in the step of forming thecoating, it is desired that the concentration of Zr is 1,000 to 10,000ppm and the concentration of F is 600 to 13,000 ppm. It is desired thatthe electrolytic treating solution has a pH of 2 to 5 and, morepreferably, 2.5 to 4. The temperature of the electrolytic treatingsolution is desirably 30 to 60° C.

Various kinds of compounds that will be described later can be added tothe electrolytic treating solution used in the step of forming thecoating. Here, the electrolytic treating solution basically containsnitric acid ions and ammonium ions for adjusting the pH as well as Feions eluted out from the base material in addition to containing Zr ionsand F ions.

There is no particular limitation on the chemicals used for forming Zrions that constitute the electrolytic treating solution, and there canbe used, for example, K₂ZrF₆, (NH₄)₂ZrF₆, (NH₄)₂ZrO(CO₃)₂, H₂ZrF₆,ZrO(NO₃)₂ and ZrO(CH₃COO)₂. In the invention, the above chemicals may beused alone or in a combination of two or more kinds.

If the coating of a Zr compound is to be formed by the cathodicelectrolytic treatment, it is, usually, desired to use a treatingsolution that contains F ions in addition to the above-mentioned Zr ionsas the electrolytic treating solution. F ions contained in theelectrolytic treating solution work as a complexing agent that enhancessolubility of Zr ions in the electrolytic treating solution. Therefore,the Zr compound can be precipitated maintaining a uniform thickness onthe base plate, and the adhesion can be further improved between thecoating and the organic resin layer.

If the electrolytic treating liquid contains F ions in small amounts,then Zr locally precipitates; i.e., the coating includes a mixture ofportions where Zr is thickly present and portions where Zr is thinlypresent. Namely, the coating lacks uniformity in the thickness and, as aresult, has poor adhesiveness and corrosion resistance after theworking. In the step of forming the coating, therefore, it is importantthat the molar ratio F/Zr of F atoms to Zr atoms in the coating is socontrolled as to be not less than 0.6.

There is no particular limitation on the chemicals used for forming Fions in the electrolytic treating solution, and there can be usedammonium zirconium fluoride, aluminum fluoride, titanium fluoride,sodium fluoride, ammonium fluoride, hydrofluoric acid, calcium fluoride,hexafluorosilicic acid, and sodium hexafluorosilicate. Among them, it isdesired to use those chemicals that are highly soluble in water.

In order to improve electric conductivity in the treating solution andto adjust the pH of the treating solution, further, the electrolytictreating solution may be added with nitric acid ions and ammonium ionsin ranges in which they do not impair the formation of the Zr compoundcoating.

To the electrolytic treating solution, furthermore, there can be addedone or more kinds of additives selected from such organic acids ascitric acid, lactic acid, tartaric acid and glycolic acid or such highmolecular compounds as polyacrylic acid, polyitaconic aid and phenolresin. Upon adding additives such as organic acid and phenol resin tothe electrolytic treating solution, additives such as organic acid andphenol resin can be contained in the Zr compound coating that is formedto thereby impart flexibility of the coating of the oxygen compound of ametal and to further improve adhesiveness to the organic resin layer.

In subjecting the base material to the cathodic electrolytic treatment,the current density is not specifically limited but is, preferably, 1 to30 A/dm².

If the base material is to be subjected to the cathodic electrolytictreatment, it is desired to employ a discrete electrolytic system whichrepeats the cycle of flowing the electric current and interrupting theelectric current. In this case, the total time for flowing the electriccurrent to the base material (total time for flowing the electriccurrent in conducting the cycle of flowing and interrupting the electriccurrent a plurality of number of times) is, preferably, 0.3 to 30seconds.

In subjecting the base material to the cathodic electrolytic treatment,further, an opposing electrode of any kind may be installed on the basematerial if it does not dissolve in the electrolytic treating solutionwhile the cathodic electrolytic treatment is being conducted. It is,however, desired to use a titanium plate coated with iridium oxide fromthe standpoint of a small oxygen overvoltage and that electrode platesparingly dissolves in the electrolytic treating solution.

<Step of Adjusting the Surfaces>

An important feature of the present invention is to conduct the step ofadjusting the surfaces by using the element of the Group II after theabove-mentioned step of forming the coating.

That is, the surface-treated steel sheet forming the surface-treatinglayer that that chiefly comprises zirconium and oxygen and containsfluorine obtained through the step of forming the coating, is subjectedto any one or more of the dip treatment, the spray treatment or thecathodic electrolytic treatment by using an aqueous solution thatcontains the element of the Group II for adjusting the surfaces. Throughthis treatment, the element of the Group II is made present in thesurface-treating layer on the surface side thereof. Here, as describedabove, it is specifically desired that fluorine reacts with the elementof the Group II and is insolubilized so as to be made present as afluorine compound.

After the treatment such as the dipping treatment, the steel sheet issqueezed with the rolls to remove the aqueous solution used foradjusting the surfaces, is washed with water, is further squeezed with,the rolls to remove the washing water and is, thereafter, dried with thehot air or the like.

If the step of adjusting the surface is not conducted, the containermade from the organic resin-coated surface-treated steel sheet appliedwith the organic resin coating permits fluorine in the coating to eluteout into the content in the step of sterilization treatment with hotwater, such as retort treatment. As a result, the coating induces achange in the structure thereof causing a decrease in the propertiessuch as corrosion resistance, close adhesion, etc.

Therefore, prior to applying the resin coating for obtaining thematerial for cans, it becomes important to insolubilize the fluorine inadvance by forming the layer of the compound thereof that has reactedwith the element of the Group II in the step of adjusting the surfaces,the fluorine having been dispersed on the surface side in thesurface-treating layer that chiefly comprises zirconium and oxygen andcontains fluorine.

As the element of the Group II used for the aqueous solution foradjusting the surfaces, there can be exemplified beryllium, magnesium,calcium, strontium, barium and radium. In using these elements for theaqueous solution for adjusting the surfaces, however, attention must begiven to that if the chemicals are soluble in water, if the chemicalseasily bond to the fluorine, if the chemicals are capable of forming asparingly soluble fluorine compound, if the chemicals are capable ofmaintaining safety and sanitation excellently, and if the chemicals arenot expensive. From the above points of view, therefore, it is desiredto use calcium or magnesium capable of forming sparingly soluble CaF₂ orMgF₂ after having reacted with fluorine.

Here, the aqueous solution for adjusting the surfaces may contain eithercalcium ions or magnesium ions, or both of them. If the ions of eitherone type are to be contained, the aqueous solution containing calciumcan be most desirably used for adjusting the surfaces.

If the aqueous solution containing calcium is used for adjusting thesurfaces, any chemicals can be used without limitation provided they candissolve in water, and there can be used calcium lactate, calciumhydroxide, calcium gluconate, calcium chloride, calcium nitrate, calciumsulfate, calcium citrate, calcium carbonate and calcium phosphatemonobasic. Among them, it is desired to use the one having largewater-solubility. If magnesium is used, further, there is no particularlimitation on the chemicals that can be used provided they dissolve inwater. Preferably, there can be used magnesium chloride, magnesiumnitrate, magnesium sulfate, magnesium citrate, magnesium acetate andmagnesium gluconate. If the aqueous solution is alkaline, magnesiumgluconate can be particularly preferably used.

The aqueous solution for adjusting the surfaces may have been containingfluorine at the time of preparing the chemicals or may have beencontaining fluorine as it is dissolved in the step of adjusting thesurfaces, as will be obvious from the fact that the aqueous solution foradjusting the surfaces has the role of insolubilizing the fluorinecontained in the surface-treating coating. The invention, however, doesnot dare to add fluorine thereto or contain fluorine therein.

In the step of adjusting the surfaces, further, it is desired that theaqueous solution for adjusting the surfaces has a pH in a range of 2 to13, preferably, 5 to 11 and, more preferably, 5.5 to 7. Within thisrange, the fluorine in the surface-treating layer assumes the state offree F ions but not complex ions, and are capable of being moreefficiently bonded to the element of the Group II that is forming thefluorine compound with the element of the Group II in thesurface-treating layer on the surface side thereof maintaining stabilityimproving, therefore, resistance against the elution of fluorine andlowering the fluorine concentration in the drain water.

If the pH is lower than 2, the peripheral equipment and the steel sheetwhich is the base material itself are adversely affected in terms ofcorrosion resistance. If the pH exceeds 11 to become alkaline, theability decreases for stably forming the fluorine compound in thesurface-treating layer on the surface side thereof. If the pH exceeds13, in particular, fluorine dissolves at an increased rate in theaqueous solution for adjusting the surfaces eventually causing anincrease in the fluorine concentration in the drain water, which is notdesirable.

As the alkaline chemicals for adjusting the pH of the aqueous solutionfor adjusting the surfaces, there can be most simply used a hydroxidecompound of the element of the Group II that exhibits alkalinity uponbeing dissolved in water, such as Ca(OH)₂ or Mg(OH)₂. These chemicals,however, dissolve in water in relatively small amounts. If the step ofadjusting the surfaces is continuously conducted by dipping or cathodicelectrolysis or if the cathodic electrolysis is conducted, therefore,the chemical must be fed frequently often requiring laborious work formaintaining and controlling the aqueous solution. In such a case, it isdesired to employ a spray system which sprays at all times a new aqueoussolution onto the steel sheet for adjusting the surfaces from thestandpoint of easy processing. Even if the chemical of the element ofthe Group II does not dissolve in water and does not produce alkalinity,the pH can be adjusted by adding a chemical that contains one or two ormore kinds of sodium, ammonium and potassium, and the solution can beused as an alkaline aqueous solution for adjusting the surfaces.

As the chemical other than the element of the Group II used foradjusting the pH of the aqueous solution for adjusting the surfaces,there can be exemplified ammonia, ammonium zirconium carbonate, sodiumhydroxide, sodium carbonate, sodium hydrogencarbonate, sodium phosphate,sodium hydrogenphosphate, potassium hydroxide, potassium carbonate,sodium borate and sodium silicate, which may be used in two or morekind.

As required, further, various surfactants and chelating agents may beadded to the aqueous solution for adjusting the surfaces.

It is desired that the ionic concentration of the element of the GroupII contained in the aqueous solution for adjusting the surfaces is in arange of 0.002 to 0.5 mols/l. If the ionic concentration is less than0.002 mol/l, the reaction efficiency becomes poor in forming thefluorine compound with the element of the Group II in thesurface-treating layer on the surface side, thereof. If the ionicconcentration exceeds 0.5 mols/l, the element of the Group IIprecipitates too much, and the cohesive force of the coating decreases,which is not desirable.

In the step of adjusting the surfaces as described above, the diptreatment, spray treatment or cathodic electrolytic treatment can beconducted by using the aqueous solution containing the element of theGroup II for adjusting the surfaces. From the standpoint of quicktreatment, it is desired to also add the cathodic electrolytic treatmentin the aqueous solution for adjusting the surfaces. The spray treatmentand the dip treatment, on the other hand, are means desirable from thestandpoint of simplicity. The aqueous solution for adjusting thesurfaces should have a pH in the above-mentioned range and,specifically, in the range of 5.5 to 7 from the standpoint of loweringthe dissolution of fluorine in the aqueous solution for adjusting thesurfaces and lowering the load exerted on the drain water.

If the cathodic electrolytic treatment is to be conducted, it is desiredthat the aqueous solution containing the element of the Group II foradjusting the surfaces has an electric conductivity of not less than 2mS/cm from the standpoint treatment efficiency.

As described above, it is made possible to make present the element ofthe Group II and, specifically, the compound of the element of the GroupII in the surface-treating layer on the surface side through the step ofadjusting the surfaces. Here, the compound of the element of the GroupII formed in the surface-treating layer on the surface side thereof is,preferably, a fluorine compound and, more preferably, is sparinglysoluble. The sparingly soluble compound formed by using the element ofthe Group II is, preferably, a compound of calcium and/or magnesium, ormay be a single compound of either calcium or magnesium, or a compoundof both calcium and magnesium. In the case of the single compound, themost desired is the fluorine compound of calcium forming the sparinglysoluble compound.

The molar ratio AE/Zr of the element AE of the Group II and zirconium.Zr in the surface-treating layer is, preferably, not less than 0.2 and,specifically, in a range of 0.4 to 1.8. It is more effective if thethickness of the element of the Group II in terms of the weight is notless than 7 mg/m² and, specifically, in a range of 7 to 150 mg/m².

In the step of adjusting the surfaces, there is no particular limitationon the temperature of the aqueous solution for adjusting the surfaces.From the standpoint of the reactivity and controlling the temperature,however, it is desired that the temperature lies in a range of 30 to 80°C. and, specifically, 30 to 60° C. Further, the total treating time suchas of the dip treatment, spray treatment and cathodic electrolytictreatment using the aqueous solution for adjusting the surfaces is in arange of 0.1 to 5 seconds and, more preferably, 0.5 to 3 seconds.

After the treatment with the aqueous solution for adjusting the surfacesin the step of adjusting the surfaces, it is also allowable to add thewashing treatment by dipping in, or spraying with, the warm water or hotwater heated at about 40° C. to about 95° C.

(Steel Sheet as the Base Material)

As the steel sheet for use as the surface-treated steel sheet of thepresent invention, there can be used, for example, a hot-rolled steelsheet based on a continuously casted aluminum killed steel, acold-rolled steel sheet obtained by cold-rolling the hot-rolled steelsheet, and a steel sheet obtained by plating metals inclusive of Zn, Sn,Ni, Cu, Al, etc. on the hot-rolled steel sheet or the cold-rolled sheet.

It is, further, allowable to use a steel sheet having, on part or on thewhole surface thereof, an alloy layer such as of an Sn—Ni—Fe alloy, anSn—Fe alloy or an Ni—Fe alloy, as well as a steel sheet having a layerof a metal such as Sn or Ni, further, plated on the above alloy layer.Among them, a steel sheet is most desirably used as the base materialwithout having a metal-plated layer or having a metal-plated layer butpermitting iron to be locally exposed in a dispersed manner from thestandpoint of cost.

The thickness of the base material is not specifically limited and maybe suitably selected depending on the use, but is, preferably, 0.07 to0.4 mm.

(Organic Resin Coating)

As described above, the surface-treated steel sheet obtained by thepresent invention has an organic resin coating formed on thesurface-treating layer. The organic resin coating excellently adheres tothe organic resin layer. Even in case the surface-treated steel sheet isretort-treated, the organic resin coating prevents the organic resinlayer from peeling. The organic resin coating, further, effectivelyprevents the corrosion from proceeding even in case the organic resinlayer is cracked and the metal surface is exposed in wet environmentand, therefore, suppresses the metal components constituting thecontainer from eluting out.

The resin that constitutes the organic resin coating is not specificallylimited and may be suitably selected depending on the use of thesurface-treated steel sheet of the invention (depending on the use suchas cans and containers for containing specific contents). Namely, therecan be exemplified resin coatings made from various thermoplastic resinsand films made from thermosetting coating materials or thermoplasticcoating materials. As the resin coating made from the thermoplasticresin, there can be exemplified olefin resin films such as ofpolyethylene, polypropylene, ethylene-propylene copolymer,ethylene-vinyl acetate copolymer, ethylene-acrylic ester copolymer andionomer; polyester films such as of polyethylene terephthalate andpolybutylene terephthalate; polyamide films such as nylon 6, nylon 6,6,nylon 11, and nylon 12; and thermoplastic resin films such as polyvinylchloride film and polyvinylidene chloride film, which may not have beenstretched or may have been biaxially stretched. Among them, particularlypreferred is an unoriented polyethylene terephthalate obtained bycopolymerizing an isophthalic acid. The resins for constituting theorganic resin coating may be used in a single kind or in a blend ofdifferent resins.

If the thermoplastic resin coating is formed as the organic resincoating, the coating may be of a single resin layer or a multiplicity oflayers formed by the simultaneous extrusion. Multiplicity of polyesterresin layers offer such an advantage that a polyester resin havingexcellent adhesiveness can be used as the underlying layer, i.e., on theside of the surface-treated steel sheet and that a polyester resinhaving resistance against the content, i.e., having resistance againstbeing extracted or having property of not adsorbing flavor component canbe used as the surface layer.

Examples of the multiplicity of polyester resin layers include, beingexpressed as surface layer/lower layer, polyethyleneterephthalate/polyethylene terephthalate.isophthalate; polyethyleneterephthalate/polyethylene.cyclohexylenedimethylene.terephthalate;polyethylene terephthalate having a small isophthalatecontent.isophthalate/polyethylene terephthalate having a largeisophthalate content isophthalate; polyethyleneterephthalate.isophthalate/[blend of polyethyleneterephthalate.isophthalate and polybutylene terephthalate.adipate] andthe like, to which only, however, the invention is in no way limited. Itis desired that the thickness ratio of the surface layer:lower layer isin a range of 5:95 to 95:5.

The above organic coatings can be blended with known blending agents forresins, such as anti-blocking agent or amorphous silica, inorganicfiller, various antistatic agents, lubricant, antioxidant andultraviolet-ray absorber according to a known recipe.

Among them, it is desired to use a tocopherol (vitamin E). It hasheretofore been known that the tocopherol is used as an antioxidant andworks to prevent a decrease in the molecular weight caused by theoxidation and decomposition when the polyester resin is beingheat-treated and works to improve resistance against being dented.Specifically, if added to a polyester composition obtained by blendingthe polyester resin with the above ethylene type polymer as a resinreforming component, the tocopherol not only provides resistance againstbeing dented but also works to prevent corrosion from occurring due tocracks formed in the coating as a result of being subjected to severeconditions during the retort sterilization and stored in a hot vendingmachine, offering an effect of greatly improving the corrosionresistance.

The tocopherol is added in an amount of, desirably, 0.05 to 3% by weightand, specifically, 0.1 to 2% by weight.

The organic resin coating applied to the surface-treated steel sheetobtained by the present invention, in the case of the thermoplasticresin coating, has a thickness in a range of, usually, 3 to 50 μm and,specifically, 5 to 40 μm and, in the case of a film, has a thickness ina range of 1 to 50 μm and, specifically, 3 to 30 μm after fired. If thethickness is smaller than the above range, corrosion resistance becomesinsufficient. If the thickness exceeds the above range, on the otherhand, problems tend to occur in regard to workability.

The surface-treated steel sheet obtained by the present invention can becoated with the organic resin by any means such as, in the case of thethermoplastic resin coating, an extrusion-coating method, a cast filmheat-adhesion method or a biaxially stretched film heat-adhesion method.In the case of the extrusion-coating method, the polyester resin in amolten state is extruded onto the surface-treated steel sheet and isthermally adhered thereto. Namely, the polyester resin is melt-kneadedby an extruder, extruded into the form of a thin film through a T-die,and the molten resin film that is extruded is passed together with thesurface-treated steel sheet through a pair of laminating rolls so as tobe pressed together into a unitary structure under cold conditionfollowed by quenching. If a multiplicity of polyester resin layers areto be extruded, use is made of an extruder for extruding the surfaceresin layer and an extruder for extruding the lower resin layer. Theflows of resins from these extruders are met together in a multi-layerdie. Thereafter, the resultant flow of resins may be extruded like inthe case of extruding the single resin layer. Further, by passing thesurface-treated steel sheet between the pair of laminating rolls in avertical direction and by feeding the webs of molten resins to bothsides thereof, it is made possible to coat both surfaces of the basematerial with the polyester resins.

Concretely described below is the production of the organic resin-coatedsurface-treated steel sheet having an organic coating of polyester resinbased on the extrusion-coating method. The surface-treated steel sheet,as required, is preheated by a heating device, and is fed to a nippingposition between the pair of laminating rolls. The polyester resin, onthe other hand, is pushed into the form of a thin film through the diehead of the extruder, fed into between the laminating rolls and thesurface-treated steel sheet, and is press-adhered onto thesurface-treated

steel sheet by the laminating rolls. The laminating rolls are maintainedat a predetermined temperature. Thin films of the thermoplastic resinsuch as polyester are pressed onto the surface-treated steel sheet andare thermally adhered thereto followed by cooling from both sidesthereof so as to obtain an organic resin-coated surface-treated steelsheet. Usually, the organic resin-coated surface-treated steel sheetthat is formed is, further, introduced into a water tank for cooling,and is quenched therein to prevent thermal crystallization.

In the extrusion-coating method, the polyester resin layer assumes thecrystallinity of a low level, i.e., has a density which is differentfrom the amorphous density thereof by not more than 0.05 g/cm³ due tothe resin composition that is selected and due to quenching by the rollsand in the cooling tank. Therefore, the polyester resin layer issufficiently guaranteed for its workability in the subsequent steps offorming cans and lids. The quenching operation is not limited to theabove example only but may be to spray the cooling water onto theorganic resin-coated surface-treated steel sheet that is formed so as toquench the laminated sheet.

The polyester resin is thermally adhered to the surface-treated steelsheet by utilizing the quantity of heat possessed by the molten resinlayer and the quantity of heat possessed by the surface-treated steelsheet. A proper range of the temperature (TI) for heating thesurface-treated steel sheet is, usually, 90° C. to 290° C. and,specifically, 100° C. to 280° C. while a proper range of the temperatureof the laminating rolls is 10° C. to 150° C.

The organic resin coating can also be formed on the surface-treatedsteel sheet obtained by the production method of the present inventionby thermally adhering, onto the surface-treated steel sheet, a polyesterresin film that is formed in advance by a T-die method or an inflationmethod. As the film, there can be used an unstretched film formed by acast-forming method by quenching the film that is extruded. It is,further, allowable to use a biaxially stretched film obtained bybiaxially stretching the film sequentially or simultaneously at astretching temperature, and thermally setting the film after having beenstretched.

(Metal Containers)

As for the metal container (can body) formed by using thesurface-treated steel sheet of the invention, it is desired that thecontainer is formed by using the organic resin-coated surface-treatedsteel sheet obtained by coating the surfaces of the surface-treatedsteel sheet with the organic resin as described earlier relying on anycan-producing method. Concretely speaking, the organic resin-coatedsurface-treated steel sheet can be used for forming a three-piece can(welded can) having a seam on the side surface thereof and a seamlesscan (two-piece can). From the standpoint of close adhesion to theorganic resin as described above, however, the surface-treated steelsheet containing Zr in large amounts is most desirably used for formingseamless cans.

The seamless can is produced relying on a conventional means such asdraw working, draw redraw working, bend-elongation working (stretching)based on the draw•redrawing, bend-elongation•ironing working based onthe draw•redrawing, or draw•ironing working in a manner that the organicresin coating is on the inner surface side of the cans.

When it comes to a seamless that is subjected to a high degree ofworking such as bend-elongation working (stretching) based on thedraw•redrawing, bend-elongation•ironing working based on thedraw•redrawing or the like, it is desired that the organic resin coatingis a thermoplastic resin coating formed by the extrusion-coating method.The organic resin-coated surface-treated steel sheet features excellentclose adhesion during the working. Namely, the coating remainsexcellently adhered even if it is subjected to severe working, and makesit possible to provide a seamless can having excellent corrosionresistance.

(Lids)

The can lid formed by using the surface-treated steel sheet of theinvention is, desirably, formed by using the organic resin-coatedsurface-treated steel sheet like the metal container described above,and is formed by a known lid-forming method. Concretely, the lids may bea flat lid, an easy-open can lid of the stay-on-tub type, and aneasy-open can lid of the full-open type.

According to the invention, the can lids of a variety of types can beformed without limitation by using the organic resin-coatedsurface-treated steel sheet of the present invention.

EXAMPLES

The invention will now be concretely described by way of Examples towhich only, however, the invention is in no way limited. The materialsto be coated, dewaxing agents and organic coatings are those arbitrarilyselected from those placed in the market, and are not to imposelimitation on the process for producing the surface-treated steel sheetof the present invention.

The process for producing the surface-treated steel sheet and themethods of evaluating the properties thereof are as described below.

(Step of Forming the Coating)

As a starting steel sheet, use was made of a low-carbon steel sheet0.225 mm in thickness and 200 mm in width. Next, as a pre-treatment, thesteel sheet was dewaxed by the electrolysis with an alkali and waswashed with an acid by being dipped in sulfuric acid. Thereafter, thesteel sheet was dipped in an electrolytic treating solution and wascathodically and electrolytically treated so that the steel sheet wascoated on its both surfaces with a compound that chiefly comprised of Zrand contained F. Next, the steel sheet was squeezed with the rolls,washed with water and, further, squeezed with the rolls to remove thewashing water to thereby form the coatings.

Electrolytic Treating Solution:

-   -   An aqueous solution in which ammonium zirconium fluoride was        dissolved as a Zr compound, the concentration of Zr being 6,000        ppm and the concentration of F being 7,500 ppm.

pH of the Electrolytic Treating Solution:

-   -   3.0 (pH was adjusted with nitric acid and/or ammonia).

Temperature of the Electrolytic Treating Solution:

-   -   40° C.

Opposing Electrode:

-   -   Titanium plate coated with iridium oxide.

Method of Flowing Electric Current During the Cathodic Electrolysis:

-   -   The electric current was flown one time or a plurality of times        (hereinafter called number of cycles) at a current density of 3        A/dm² for 0.15 seconds each time.

(Step of Adjusting the Surfaces)

The steel sheet after the step of forming the coating was treated withthe aqueous solution for adjusting the surfaces for a predeterminedperiod of time, squeezed with the rolls, washed with water, further,squeezed with rolls and was, thereafter, dried with the hot air toobtain a surface-treated steel sheet.

In the step of adjusting the surfaces according to the presentinvention, there can be conducted any one or more of the dip treatment,spray treatment and cathodic electrolytic treatment by using the aqueoussolution containing the element of the Group II for adjusting thesurfaces. In Examples of the invention, however, there were conductedthe dip treatment, the spray treatment and the cathodic electrolytictreatment by using the aqueous solution containing calcium or magnesiumfor adjusting the surfaces. In the cathodic electrolysis in the step ofadjusting the surfaces, a titanium plate coated with iridium oxide wasused as the opposing electrode, and the electric current-flowing cyclewas repeated a plurality of times, each cycle comprising flowing theelectric current for 0.15 seconds followed by the interruption of 0.1second.

(Producing the Organic Resin-Coated Surface-Treated Steel Sheets)

An organic resin-coated surface-treated steel sheet was obtained bythermally adhering a 19 μm-thick stretched film a polyethyleneterephthalate/isophthalate copolymer composition containing 11 mol % ofisophthalic acid component onto one surface, that becomes the innersurface of the can, of the surface-treated steel sheet obtained aboveand by thermally adhering a 13 μm-thick stretched film of a polyethyleneterephthalate/isophthalate copolymer composition containing 12 mol % ofisophthalic acid component and, further, containing titanium oxide andcolored white onto the other surface that becomes the outer surface ofthe can, by using laminating rolls followed readily by cooling withwater while paying attention such that the film was oriented to asuitable degree. The obtained organic resin-coated surface-treated steelsheet was partly used for evaluating cross-cut corrosion resistance, butthe rest of it was used for producing metal cans.

(Producing the Metal Cans)

Paraffin wax was electrostatically applied onto both surfaces of theorganic resin-coated surface-treated steel sheet obtained above. Thesteel sheet was punched into a circle 143 mm in diameter and wasdraw-formed into a cup 91 mm in diameter and 36 mm in height accordingin a customary manner. The draw-formed cup was at the same timesubjected to the draw-ironing working repetitively two times to form acup having a small diameter and a large height. The thus obtained cuppossessed properties as described below.

-   -   Diameter of cup: 52.0 mm    -   Height of cup: 111.7 mm    -   Reduction ratio of sheet thickness in the can wall    -   relative to the initial sheet thickness: 30%

After the doming, the cup was heat-treated at 220° C. for 60 seconds toremove strain from the resin film, followed by trimming for the openend, printing on the curved surface, necking into a diameter of 50.8 mmand flanging to thereby obtain a seamless can having a capacity of 200ml.

(Measuring the Amount of Zr and the Amount of AE (Amount of the Elementof the Group II))

By using an X-ray fluorometric analyzer (Model: ZSX100e manufactured byRigaku Co.), the surface-treated steel sheet obtained above was measuredfor its amount of Zr and the amount of AE (amount of Ca or amount of Mgin Examples) contained in the metal compound coating. The molar ratioAE/Zr was found according to the following formula,

AE/Zr=(amount of AE/atomic weight of AE)/(amount of Zr/atomic weight ofZr)

(Measuring the Amount of F)

Microanalysis of the amount of F in the obtained surface-treated steelsheet based on the X-ray fluorometry poses limitation in regard toquantitative precision. Specifically, it is difficult to determine theamount of F from the surface-treated steel sheet containing Fin amountsof less than 1.5 mg/m². After having studied variously, therefore, wehave measured the amount of F in a manner as described below. That is,by using a special cell capable of holding 160 cm² of one surface of thesurface-treated steel sheet in a state of being contacted to 183 g ofvery pure water, the surface-treated steel sheet was retort-treated at130° C. for 30 minutes. Thereafter, fluorine ions released into verypure water were measured by the ion chromatography (DX-320 manufacturedby DIONEX Co.). The amount of F present in very pure water was foundfrom the obtained concentration of F and was converted into the amountof F present in the surface-treated steel sheet per a unit area, and wasregarded to be the amount of F in the coating.

Fluorine does not almost elute out despite the surface-treated steelsheets that have passed through the step of adjusting the surfaces shownin Examples are subjected to the retort-treatment and, therefore, theamounts of F in the surface-treated steel sheets cannot be known. Forthe surface-treated steel sheets that have passed through the step ofadjusting the surfaces, therefore, the amounts of F were measured by theX-ray fluorometry. In case the amount of F was less than 1.5 mg/m²,however, the peak in the X-ray fluorometry was not clear. Therefore, thescraped powder of the surface-treating coating was collected in amountsequivalent to more than 10 times the areas that are usually measured bythe X-ray fluorometry, and fluorine was measured by the X-rayfluorometry and was converted into an amount thereof per a unit area.

(Measuring the Reduction Ratio of F)

The percentage reduction in the amount of F in the surface-treated steelsheet that has passed through the step of adjusting the surface wasfound from the amount of F in the surface-treated steel sheet that wasformed through only the step of forming the coating. The evaluationthereof serves as an index of fluorine load exerted on the drain waterin the step of adjusting the surfaces. It is desired that the index isnot more than 30%.

(Evaluating the Cross-Cut Corrosion Resistance)

By using a cutter knife, a portion of the obtained surface-treated steelsheet that would become the inner surface side of a can was engravedover a length of 4 cm in a crossing manner deep enough to reach thesteel sheet to thereby prepare a test piece. The test piece was put in abottle and was dipped in a commercially available coffee (trade name,Blendy, bottled coffee, low sugar, produced by Ajinomoto General FoodsCo.) The bottle was deaerated, stored at 37° C. for 4 weeks to evaluatethe state of corrosion. During this period, the coffee was regularlyrenewed to suppress the generation of mold as much as possible. Thecorroded state was evaluated by taking the test piece out of the coffee.Namely, the cross-cut portion and the surroundings thereof were observedwith the eye in regard to if the organic resin layer was peeled or ifthe color has changed due to the formation of corroded product.

A test piece whose color has changed or whose film has peeled by amaximum width of not less than 3 mm around the cross-cut portion wascounted to be one point, a test piece having a maximum width of peelingof not less than 2 mm but less than 3 mm was counted to be two points, atest piece of not less than 1 mm but less than 2 mm was counted to bethree points, a test piece of not less than 0.5 mm but less than 1 mmwas counted to be four points, and a test piece of less than 0.5 mm wascounted to be five points. Test pieces of counts of three or more pointswere regarded to be acceptable.

(Evaluating the Adhesion of the Resin to the Inner Surface of the Can)

A seamless can that was obtained was filled with distilled water,double-wrap-seamed with a lid, and was retort-treated at 125° C. for 30minutes. Thereafter, the lid was removed from the can body, the contentwas removed from the can, and the can was cut into halves with thedirection of rolling the surface-treated steel sheet at 45 degrees as aboundary. Next, the can cut into halves was dipped in a solutionobtained by adding 0.02% by weight of a surfactant to an aqueoussolution containing 1% by weight of sodium chloride for one hour. Byusing a pair of scissors, the can was further cut into halves from theside of the can bottom with the rolling direction of 135 degrees as aboundary. The cross section of a radial portion of the bottom of thefinally cut can on the inner surface side thereof was observed in regardto if the resin was peeled to thereby evaluate the close adhesion of theresin. The can with peeling of not less than 10 mm near the cut surfacewas counted to be one point, the can with peeling of less than 10 mm butnot less than 5 mm was counted to be two points, the can with peeling ofless than 5 mm but not less than 2 mm was counted to be three points,the can with peeling but less than 2 mm was counted to be four points,and the can with no peeling was counted to be five points. The cans ofcounts of three or more points were regarded to be acceptable.

(Evaluating the Resistance Against the Elution of F)

The obtained seamless can was filled with 183 g of very pre water,double-seamed, and was retort-treated at 130° C. for 30 minutes.Thereafter, fluorine ions released into very pure water was measured bythe ion chromatograph (DX-320 manufactured by DIONEX Co.). The cansreleasing F by not less than 0.1 ppm were evaluated to be X and the cansreleasing F by less than 0.1 ppm were evaluated to be ◯.

(Evaluating the Load Exerted on the Drain Water)

The load exerted on the drain water was evaluated from the reductionratio of F. The cases of when the reduction ratios of the amounts of Fwere not more than 30% were evaluated to be ◯ and the cases of when thereduction ratios thereof were not less than 30% were evaluated to be Δ.◯ is preferred to Δ.

Example 1

In the step of forming the coating, first, a titanium plate coated withiridium was used as the opposing electrode in the electrolytic treatingsolution, a steel sheet was used as the cathode, an electric current wasflown one time at a current density of 3 A/dm² for 0.15 seconds, thesteel sheet was squeezed with the rolls to remove the electrolytictreating solution, washed with water of normal temperature and was,further, squeezed with the rolls to remove the washing water. Next, thesurfaces were adjusted by the cathodic electrolytic treatment. As theaqueous solution for adjusting the surfaces, use was made of an aqueoussolution containing calcium lactate in an amount of 0.1 mol/l, andhaving an electric conductivity of 6.57 mS/cm and a pH of 6.96. In theaqueous solution maintained at a liquid temperature of 30° C. foradjusting the surfaces, the step of adjusting the surfaces was conductedby repeating twice the cycle of flowing an electric current at a currentdensity of 4 A/dm² for 0.15 seconds followed by an interruption of 0.1second. The steel sheet after the step of adjusting the surfaces wassqueezed with the rolls to remove the aqueous solution, washed withwater, squeezed again with the rolls to remove the washing water, andwas dried to obtain a surface-treated steel sheet.

The obtained surface-treated steel sheet was measured for the amount ofZr, amount of AE and amount of F by the methods described above.Measured amounts of the coating were as shown in Table 1 which alsoshows a molar ratio AE/Zr of the element (AE) of the Group II and Zrcalculated from the amounts of the coating, and the reduction ratio ofF. In Table, “-” stands for that the measurement was not taken.

Example 2

A surface-treated steel sheet was obtained in the same manner as inExample 1 but flowing the electric current at a current density of 10A/dm² and repeating the cycle twice in the step of forming the coating,and flowing the electric current at a density of 1 A/dm² in the step ofadjusting the surfaces.

Example 3

A surface-treated steel sheet was obtained in the same manner as inExample 2 but flowing the electric current at a density of 6.5 A/dm² inthe step of adjusting the surfaces.

Example 4

A surface-treated steel sheet was obtained in the same manner as inExample 2 but dip-treating the steel sheet at 60° C. for 3 seconds inthe step of adjusting the surfaces.

Example 5

A surface-treated steel sheet was obtained in the same manner as inExample 3 but flowing the electric current at a density of 10 A/dm² andrepeating the cycle 4 times in the step of forming the coating.

Example 6

A surface-treated steel sheet was obtained in the same manner as inExample 5 but preparing an aqueous solution for adjusting the surfaceshaving a pH of 11.0 and an electric conductivity of 7.13 mS/cm by addingammonia to an aqueous solution containing calcium lactate in an amountof 0.1 mol/l, and conducting the step of adjusting the surfaces byrepeating two times the cycle of cathodic electrolysis at a liquidtemperature of 40° C. and flowing the electric current at a density of 2A/dm².

Example 7

A surface-treated steel sheet was obtained in the same manner as inExample 6 but conducting the cathodic electrolysis by flowing a currentat a density of 7 A/dm² in the step of adjusting the surfaces.

Example 8

A surface-treated steel sheet was obtained in the same manner as inExample 6 but spray-treating the steel sheet for 3 seconds in the stepof adjusting the surfaces.

Example 9

A surface-treated steel sheet was obtained in the same manner as inExample 2 but repeating the cycle 8 times in the step of forming thecoating.

Example 10

A surface-treated steel sheet was obtained in the same manner as inExample 9 but flowing the electric current at a density of 4 A/dm² andrepeating the cycle 4 times in the step of adjusting the surfaces.

Example 11

A surface-treated steel sheet was obtained in the same manner as inExample 10 but flowing the electric current at a density of 6.5 A/dm² inthe step of adjusting the surfaces.

Example 12

A surface-treated steel sheet was obtained in the same manner as inExample 11 but preparing an aqueous solution for adjusting the surfaceshaving an electric conductivity of 15.1 mS/cm and a pH of 5.61 by usinga magnesium nitrate hexahydrate in an amount of 0.1 mol/l, andconducting the step of adjusting the surfaces by repeating two times thecycle of cathodic electrolysis at a liquid temperature of 40° C. andflowing the electric current at a density of 4 A/dm².

Example 13

A surface treated steel sheet was obtained in the same manner as inExample 10 but repeating the cycle 12 times in the step of forming thecoating.

Example 14

A surface treated steel sheet was obtained in the same manner as inExample 11 but repeating the cycle 12 times in the step of forming thecoating.

Example 15

A surface-treated steel sheet was obtained in the same manner as inExample 13 but using an aqueous solution containing calcium nitrate inan amount of 0.1 mol/l and having a pH of 5.6 as the aqueous solutionfor adjusting the surfaces and repeating two times the cycle of cathodicelectrolysis at a liquid temperature of 40° C. and flowing the electriccurrent at a density of 4 A/dm² in the step of adjusting the surfaces.

Comparative Example 1

A surface-treated steel sheet was obtained in the same manner as inExample 1 but without conducting the step of adjusting the surfaces.

Comparative Example 2

A surface-treated steel sheet was obtained in the same manner as inExample 2 but without conducting the step of adjusting the surfaces.

Comparative Example 3

A surface-treated steel sheet was obtained in the same manner as inExample 5 but without conducting the step of adjusting the surfaces.

Comparative Example 4

A surface-treated steel sheet was obtained in the same manner as inExample 9 but without conducting the step of adjusting the surfaces.

Comparative Example 5

A surface-treated steel sheet was obtained in the same manner as inExample 13 but without conducting the step of adjusting the surfaces.

TABLE 1 Step of forming the Step of adjusting surfaces coating Dip orCurrent Element Temp. of Current spray density (AE) of aq. sol. densitytime, Method A/dm² Cycles Method Group II ° C. pH A/dm² Cycles sec. Ex.1 *1 3 1 *1 Ca 30 6.96 4 2 — Ex. 2 *1 10 2 *1 Ca 30 6.96 1 2 — Ex. 3 *110 2 *1 Ca 30 6.96 6.5 2 — Ex. 4 *1 10 2 dipped Ca 60 6.96 — — 3 Ex. 5*1 10 4 *1 Ca 30 6.96 6.5 2 — Ex. 6 *1 10 4 *1 Ca 40 11 2 2 — Ex. 7 *110 4 *1 Ca 40 11 7 2 — Ex. 8 *1 10 4 sprayed Ca 40 11 — — 3 Ex. 9 *1 108 *1 Ca 30 6.96 1 2 — Ex. 10 *1 10 8 *1 Ca 30 6.96 4 4 — Ex. 11 *1 10 8*1 Ca 30 6.96 6.5 4 — Ex. 12 *1 10 8 *1 Mg 40 5.61 4 2 — Ex. 13 *1 10 12*1 Ca 30 6.96 4 4 — Ex. 14 *1 10 12 *1 Ca 30 6.96 6.5 4 — Ex. 15 *1 1012 *1 Ca 40 5.6 4 2 — Film composition Film composition on after coatingis surface-treated steel formed sheet Amount Amount Amount Amount AmountAmount AE/Zr of Zr of F of AE of Zr of F of AE Mole mg/m² mg/m² mg/m²mg/m² mg/m² mg/m² ratio *2 *3 *4 *5 *6 Ex. 1 12 0.8 1.4 12 0.5 15.0 2.8438 3 3 ◯ Δ Ex. 2 40 5.2 2.3 40 4.6 13.8 0.78 12 5 3 ◯ ◯ Ex. 3 35 5.2 2.335 1.3 12.6 0.83 76 5 4 ◯ Δ Ex. 4 39 5.2 2.3 39 0.4 7.7 0.45 92 5 4 ◯ ΔEx. 5 57 7.2 2.8 57 1.0 27.7 1.11 86 5 5 ◯ Δ Ex. 6 59 7.2 2.8 59 3.042.4 1.63 59 5 6 ◯ Δ Ex. 7 61 7.2 2.8 61 4.2 46.3 1.73 42 5 5 ◯ Δ Ex. 855 7.2 2.8 55 1.9 31.9 1.32 74 4 4 ◯ Δ Ex. 9 117 16.8 4.2 117 12.3 25.00.49 27 5 5 ◯ ◯ Ex. 10 115 16.8 4.2 115 11.9 57.7 1.14 29 5 5 ◯ ◯ Ex. 11115 16.8 4.2 115 12.0 69.5 1.37 29 5 5 ◯ ◯ Ex. 12 115 16.8 1.5 115 13.018.0 0.59 23 5 5 ◯ ◯ Ex. 13 181 21.2 5.1 181 17.9 70.8 0.89 15 5 5 ◯ ◯Ex. 14 169 19.8 4.8 169 17.2 91.9 1.23 13 5 5 ◯ ◯ Ex. 15 182 21.2 5.1182 19.8 141 1.77 7 5 4 ◯ ◯ Step of forming the Step of adjustingsurfaces coating Dip or Current Element Temp. of Current spray density(AE) of aq. sol. density time, Method A/dm² Cycles Method Group II ° C.pH A/dm² Cycles sec. Comp. *1 3 1 none Ex. 1 Comp. *1 10 2 none Ex. 2Comp. *1 10 4 none Ex. 3 Comp. *1 10 8 none Ex. 4 Comp. *1 10 12 noneEx. 5 Film composition Film composition on after coating issurface-treated steel formed sheet Amount Amount Amount Amount AmountAmount AE/Zr of Zr of F of AE of Zr of F of AE Mole mg/m² mg/m² mg/m²mg/m² mg/m² mg/m² ratio *2 *3 *4 *5 *6 Comp. 12 0.8 1.4 12 0.8 1.4 0.02— 2 1 ◯ X Ex. 1 Comp. 35 5.2 2.3 35 5.2 2.3 0.13 — 2 1 ◯ X Ex. 2 Comp.57 7.2 2.8 57 7.2 2.8 0.10 — 2 2 ◯ X Ex. 3 Comp. 115 16.8 4.2 115 16.84.2 0.08 — 3 3 X X Ex. 4 Comp. 181 21.2 5.1 181 21.2 5.1 0.03 — 4 3 X XEx. 5 *1: cathodic electrolysis *2: F reduction (%) in step of adjustingsurfaces, *3: Cross-cut corrosion resistance, *4: Adhesion of resin, *5:Resistance against F elusion, *6: Load on drain water

(Consideration)

As will be obvious from Table 1, in Examples 1 to 15, the amount of Zrin the coating was set to be 12 to 182 mg/m², and the steel sheets weretreated with an aqueous solution containing an element of the Group IIin the step of adjusting the surfaces to obtain the steel sheetscontaining F in amounts of 0.4 to 19.8 mg/m² in the coating. InComparative Examples 1 to 3 in which no step was conducted to adjust thesurfaces, the steel sheets were satisfactory in regard to the resistanceagainst the elution of F but were poor in regard to the cross-cutcorrosion resistance and the close adhesion. As the amount of Zrincreases, these properties are improved but the resistance against theelution of F decreases. The organic resin-coated metal sheets obtainedfrom the materials that were subjected to the step of adjusting thesurfaces of Examples 1 to 15, exhibited excellent cross-cut corrosionresistance, excellent adhesiveness on the inner surface of the metalcans, excellent resistance against the elution of F, and high degree ofadhesion of the organic resin layer. Even in case the organic resinlayer was cracked after the working for forming cans and theretort-treatment, it was confirmed that the organic resin layer remainedclosely adhered, and the containers excellently maintained the qualityof the contents.

In Examples 1 to 15, further, the reduction ratio of F was small if theamount of Zr in the coating was large. Specifically, the reduction ratioof F was not more than 30% if the amount of Zr was not less than 100mg/m². That is, it was learned that even if the reduction ratio of F wassmall, the steel sheets could be obtained having excellent corrosionresistance, close adhesion and resistance against the elution of F.

As will be obvious from Table 1, further, in Examples 1 to 15, theamounts of AE were 7.7 to 141 mg/m² whereas in Comparative Examples, theamounts of AE were 1.4 to 5.1 mg/m². In Comparative Examples, no stepwas conducted for adjusting the surfaces, and no element of the Group IIwas intentionally added to the aqueous solution or to the washing waterin the step of forming the coating. It is considered that the elementsof the Group II are stemming from Ca and Mg that were unavoidablycontained as impurities in the aqueous solution or in the washing waterin the step of forming the coating. Giving attention to the AE/Zr ratiosin Table 1, the materials of Examples 1 to 15 that exhibited favorableproperties all possessed the AE/Zr ratios of not less than 0.2 whereasthe materials of Comparative Examples 1 to 5 failing to satisfyproperties all possessed the AE/Zr ratios of less than 0.2. Therefore,use of the AE/Zr ratio makes it possible to distinguish Ca ad Mg thatare unavoidably contained as impurities.

1. A surface-treated steel sheet having a steel sheet and asurface-treating layer on at least one surface of the steel sheet, thesurface-treating layer including zirconium, oxygen and fluorine, whereinsaid surface-treating layer contains an element of the Group II on thesurface side thereof.
 2. The surface-treated steel sheet according toclaim 1, wherein the element of the Group II is present as a fluorinecompound.
 3. The surface-treated steel sheet according to claim 1,wherein the element of the Group II is at least either calcium ormagnesium.
 4. The surface-treated steel sheet according to claim 1,wherein a molar ratio AE/Zr of the element (AE) of the Group II andzirconium (Zr) in said surface-treating layer is not less than 0.2. 5.The surface-treated steel sheet according to claim 1, wherein thethickness of zirconium in terms of weight is 100 to 200 mg/m².
 6. Anorganic resin-coated surface-treated steel sheet obtained by forming anorganic resin coating on the surface-treated steel sheet of claim
 1. 7.A metal container made from the organic resin-coated surface-treatedsteel sheet of claim
 6. 8. A can lid made from the organic resin-coatedsurface-treated steel sheet of claim
 6. 9. A process for producing asurface-treated steel sheet having a steel sheet and a surface-treatinglayer on at least one surface of the steel sheet, the surface-treatinglayer including zirconium, oxygen and fluorine, said process comprisingthe steps of: forming a coating by cathodically electrolyzing the steelsheet in an aqueous solution that contains Zr ions and F ions; andthereafter, adjusting the surfaces by conducting any one or more of adip treatment, a spray treatment or a cathodic electrolytic treatment byusing an aqueous solution that contains an element of the Group II foradjusting the surface.
 10. The process for producing the surface-treatedsteel sheet according to claim 9, wherein the element of the Group II isat least one of calcium or magnesium.
 11. The process for producing thesurface-treated steel sheet according to claim 9, wherein in the step ofadjusting the surfaces, a reduction ratio of fluorine from that of thestep of forming the coating is not more than 30%.